As is well known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Importantly, olefins including ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials.
Commercial production of olefins is almost exclusively accomplished by pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Thermal cracking feed stocks include streams of ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Because of the very high temperatures employed, commercial olefin processes invariably coproduce significant amounts of acetylene and methyl acetylene. Required separation of the acetylene from the primary olefin can considerably increase the plant cost.
In a typical ethylene plant, the cracking represents about 25 percent of the cost of the unit, while the compression, heating, dehydration, recovery and refrigeration sections represent the remaining percentage of the total. This endothermic process is carried out in large pyrolysis furnaces with the expenditure of large quantities of heat, which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirements for the process are thus very large, and ways to reduce it are of substantial commercial interest. In addition, it is of significant interest to reduce the amounts of methane and heavy fuel oils produced in the cracking processor and utilize them for other than for their fuel value.
Hydrocarbon cracking is carried out using a feed which is ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil. Ethane, propane, liquid naphthas, or mixtures thereof are preferred feed to a hydrocarbon cracking unit. Hydrocarbon cracking is generally carried out thermally in the presence of dilution steam in large cracking furnaces which are heated, at least in part, by burning methane and other waste gases from the olefins process resulting in large amounts of NOx pollutants. The hydrocarbon cracking process is very endothermic and requires large quantities of heat per pound of product. However, newer methods of processing hydrocarbons utilize, at least to some extent, catalytic processes which are better able to be tuned to produce a particular product slate. The amount of steam used per pound of feed in the thermal process depends to some extent on the feed used and the product slate desired. Typically, steam pressures are in the range of about 30 lbs per sq in to about 80 lbs per sq in (psi), and amounts of steam used are in the range of about 0.2 pounds of steam per pound of feed to 0.7 pounds of steam per pound of feed. The temperature, pressure and space velocity ranges used in thermal hydrocarbon cracking processes depend to some extent upon the feed used and the product slate desired, which are well known and may be appreciated by one skilled in the art.
The type of furnace used in the thermal cracking process is also well known. However the ceramic honeycomb furnace which is described in U.S. Pat. No. 4,926,001, the contents of which patent are specifically incorporated herein by reference, is an example of a new type of cracking which could have a special utility for this process.
Several methods are known for separation of an organic gas containing unsaturated linkages from gaseous mixtures. These include, for instance, cryogenic distillation, liquid adsorption, membrane separation and the so called "pressure swing adsorption" in which adsorption occurs at a higher pressure than the pressure at which the adsorbent is regenerated. Cryogenic distillation and liquid adsorption are common techniques for separation of carbon monoxide and alkenes from gaseous mixtures containing molecules of similar size, e.g. nitrogen or methane. However, both techniques have disadvantages such as high capital cost and high operating expenses. For example, liquid adsorption techniques suffer from solvent loss and need a complex solvent make-up and recovery system.
Molecular sieves which selectively adsorb carbon monoxide from gaseous mixtures by chemisorption are also known. U.S. Pat. No. 4,019,879 and U.S. Pat. No. 4,034,065 refer to use of high silica zeolites, which have relatively high selectivities for carbon monoxide, in the pressure swing adsorption method. However, these zeolites only have moderate capacity for carbon monoxide, and more particularly require very low vacuum pressures to recover the adsorbed gases and/or to regenerate the zeolite.
U.S. Pat. No. 4,717,398 describes a pressure swing adsorption process for selective adsorption and subsequent recovery of an organic gas containing unsaturated linkages from gaseous mixtures by passing the mixture over a zeolite ion-exchanged with cuprous ions (Cu I) characterized in that the zeolite has a faujasite type crystalline structure (Y).
Kokai JP Number 50929-1968 describes a method of purifying vinyl compounds containing up to about 10 percent by weight of acetylenic compounds. In this method, acetylenic compounds were described as being adsorbed on an adsorption agent of 1-valent and/or 0-valent copper and/or silver supported on inert carrier such as .delta.-alumina, silica or active carbon. Separations described included 1000 ppm ethyl acetylene and 1000 ppm vinyl acetylene from liquid 1,3-butadiene, 100 ppm acetylene from ethylene gas, 100 ppm methyl acetylene from propylene gas, and 50 ppm phenyl acetylene from liquid styrene (vinylbenzene). Each application used fresh adsorption agent and only a short time of one hour on stream at mild conditions of temperature and pressure. Such limited applications were likely because it is well known that acetylene and these acetylene compounds react with copper and/or silver to form copper acetylide or silver acetylide. Both the acetylide of copper and silver are unstable compounds. Because they are explosive under some conditions, their possible formation presents safety problems in operation and in handling adsorbent containing such precipitates.
More recently, German Disclosure Docurnent 2059794 describes a liquid adsorption process for purification of paraffinic, olefinic and/or aromatic hydrocarbons with an adsorption agent consisting in essence of a complex of a copper (Cu I)-salt with an alkanolamine such as monoethanolamine, monoisopropanolamine, diethanolamine, triethanolamine and arylalkanolmines, and optionally in the presence of a glycol or polyglycol. However, the product stream is contaminated with unacceptable levels of components of such agents absorbed in the hydrocarbon flow. While such contamination might be removable using an additional bed of silica gel, aluminum oxide or a wide-pored molecular sieve, this would involve additional capital costs, operation expenses and perhaps safety problems.
Olefin-paraffin separations represent a class of most important and also most costly separations in the chemical and petrochemical industry. Cryogenic distillation has been used for over 60 years for these separations. They remain to be the most energy-intensive distillations because of the close relative volatilities. For example, ethane-ethylene separation is carried out at about -25.degree. C. and 320 pounds per square inch gage pressure (psig) in a column containing over 100 trays, and propane-propylene separation is performed by an equally energy-intensive distillation at about -30.degree. C. and 30 psig.
Impurity refers to compounds that are present in the olefin plant feedstocks and products. Well-defined target levels exist for impurities. Common impurities in ethylene and propylene include acetylene, methyl acetylene, methane, ethane, propane, propadiene, and carbon dioxide. Listed below are the mole weight and atmospheric boiling points for the light products from thermal cracking and some common compounds potentially found in an olefins unit. Included are some compounds which have similar boiling temperatures to cracked products and may be present in feedstocks or produced in trace amounts during thermal cracking.
______________________________________ Mole Normal Boiling Compound Weight Point, .degree. C. ______________________________________ Hydrogen 2.016 -252.8 Nitrogen 28.013 -195.8 Carbon monoxide 28.010 -191.5 Oxygen 31.999 -183.0 Methane 16.043 -161.5 Ethylene 28.054 -103.8 Ethane 30.070 -88.7 Phosphine 33.970 -87.4 Acetylene* 26.038 -84.0 Carbon dioxide* 44.010 -78.5 Radon 222.00 -61.8 Hydrogen sulfide 34.080 -60.4 Arsine 77.910 -55.0 Carbonyl sulfide 60.070 -50.3 Propylene 42.081 -47.8 Propane 44.097 -42.1 Propadiene (PD) 40.065 -34.5 Cyclo-propane 42.081 -32.8 Methyl acetylene 40.065 -23.2 Water 18.015 100.0 ______________________________________ *Sublimation temperature
Recently, the trend in the hydrocarbon processing industry is to reduce commercially acceptable levels of impurities in major olefin product streams, i.e., ethylene, propylene, and hydrogen. Need for purity improvements is directly related to increasing use of higher activity catalysts for production of polyethylene and proypropylene, and, to a limited, extent other olefin derivatives.
It is known that acetylenic impurities can be selectively hydrogenated and thereby removed from such product streams by passing the product stream over an acetylene hydrogenation catalyst in the presence of dihydrogen (molecular hydrogen, H.sub.2). However, these hydrogenation processes typically result in the deposition of carbonaceous residues or "green oil" on the catalyst which deactivates the catalyst. Therefore, acetylene hydrogenation processes for treating liquid or liquefiable olefins and diolefins typically include an oxygenation step or a "burn" step to remove the deactivating carbonaceous residues from the catalyst, followed by a hydrogen reduction step to reactivate the hydrogenation catalyst. For example, see U.S. Pat. No. 3,755,488 to Johnson et al., U.S. Pat. No. 3,792,981 to Hettick et al., U.S. Pat. No. 3,812,057 to Morgan and U.S. Pat. No. 4,425,255 to Toyoda. However, U.S. Pat. Nos. 3,912,789 and 5,332,705 state that by using selected hydrogenation catalysts containing palladium, at least partial regeneration can be accomplished using a hydrogenation step alone at high temperatures (600.degree. to 700.degree. F.) and in the absence of an oxygenation step.
Selective hydrogenation of the about 2000 to 4000 parts per million of acetylenic impurities to ethylene is generally a crucial operation for purification of olefins produced by thermal steam cracking. Typical of a small class of commercially useful catalysts are materials containing very low levels of an active metal supported on an inert carrier, for example a particulate bed having less than about 0.03 percent (300 ppm) palladium supported on the surface skin of carrier pellets having surface area of less than about 10 m.sup.2 /gm.
Many commercial olefin plants using steam crackers use front-end acetylene converters, i.e. the hydrogenation unit is fed C.sub.3 and lighter cracked gas, which feed has a high enough concentration of hydrogen to easily hydrogenate the acetylenic impurities; however, when run improperly, will also hydrogenate a large fraction of the ethylene and propylene product. Both hydrogenation of acetylene and ethylene are highly exothermic as shown below:
______________________________________ C.sub.2 H.sub.2 + H.sub.2 .fwdarw. C.sub.2 H.sub.4 H = -41 kcal/mole C.sub.2 H.sub.4 + H.sub.2 .fwdarw. C.sub.2 H.sub.6 H = -32.7 kcal/mole ______________________________________
Accelerated catalyst deactivation and thermal runaways caused by loss in catalyst selectivity are common problems which plague acetylene converters. Such problems result in unscheduled shutdowns and increased costs to replace deactivated catalyst.
The problem of over-hydrogenation is aggravated because the rate constant for ethylene hydrogenation to ethane is 100 times faster than for the hydrogenation of acetylene to ethylene. As a means to avoid a C.sub.2 H.sub.4 hydrogenation thermal runaway, acetylene, carbon monoxide and diolefins concentrations must be high enough to cover most active sites so none are left to adsorb ethylene. For example, acetylene, carbon monoxide, methyl acetylene, and propadiene have bond strengths to palladium which are stronger than the ethylene to palladium bonds. Selection of active metal, size of the metal particles and other physical and chemical factors ultimately affect the "operating temperature window" which is the delta of temperature between acetylene conversion to ethylene (typically in a range from about 100.degree. F. to about 150.degree. F.) and thermal runaway where all molecular hydrogen is converted and a large amount of the ethylene is converted to ethane (about 170.degree. F. to about 225.degree. F.). The wider the window, the safer is operation of the unit.
It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods for production of unsaturated hydrocarbons, e.g. olefins, from thermal cracking of hydrocarbon feed stocks, which olefin can be used for manufacture of polymeric materials using higher activity catalysts.
More particularly, it is an object of the present invention to provide an improved method for purification of ethylene and/or propylene containing small amounts of acetylenic impurities, carbon oxides and/or other organic components that are impurities in olefinic process streams, by passing the impure olefin stream through a particulate bed of heterogeneous adsorbent comprising a metal, supported on a high surface area carrier, under conditions suitable for reversible adsorption of alkynes.
It is another object of the present invention to provide an improved aforesaid purification method that employs an adsorbent that, even after a substantial period of aging, exhibits an ability to withstand frequent regenerations and yet retain useful adsorption capacity.
It is further an object of this invention to provide an improved process for regeneration of adsorbent loaded with acetylenic impurities.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.